Conventional writing paper is typically formed of a thin layer of interchangeable cellulosic fibers and is produced from raw materials such as wood, pulp, textile fibers, waste materials and waste paper. One major drawback associated with conventional writing paper is its low physical strength when wet. This is believed to be attributable to the abundance of hydroxyl groups in the cellulose which imparts pronounced wetability to the paper when exposed to, for example, water. In addition to this disadvantage, the cost of generating pulp for conventional writing paper has dramatically increased over the years.
In view of these drawbacks, numerous attempts have been made in the industry to produce paper substitutes from synthetic materials such as synthetic resins. Although various synthetic olefin resins, such as polyethylene and polypropylene, have been proposed as the bases for paper substitutes, the surfaces of such olefin resins are inherently hydrophobic and non-polar, resulting in poor or inadequate receptivity to common writing and printing materials such as inks, pencil leads, paint and the like. Attempts to improve the ink-receptivity of such olefin resins by the addition of polar fillers, for example, have been unsuccessful by reason of the fact that relatively small amounts of such fillers are believed to be ineffective since they remain totally encapsulated by the olefin resins and thus the surface of the sheets still present an overall homogeneous non-polar surface. Large amounts of such fillers, on the other hand, are believed to result in a deterioration of the desired physical properties, such as flexibility, tear resistance, tensile strength, etc., to the point where the filled olefin resins are no longer useful as paper substitutes.
In attempts to overcome these deficiencies, the industry has resorted to a variety of elaborate and complicated methods to produce synthetic papers. For example, U.S. Pat. No. 3,775,521 discloses a thermoplastic synthetic paper in the form of a cellular multi-layer film formed by biaxially stretching a thermoplastic sheet composed of an olefin resin, a styrene resin and an inorganic filler. U.S. Pat. No. 3,775,521 teaches that in order to impart writeability and printability, it is critical to mechanically treat the synthetic papers, i.e., to stretch the synthetic papers, to generate mechanically produced microvoids. For examples of other synthetic papers which rely upon mechanical processing, i.e., stretching, to produce microvoids to impart writeability and printability thereto, see U.S. Pat. Nos. 4,705,719, 4,359,497, 4,340,639, 4,318,950, 4,151,159, 4,097,645, 3,922,427, 3,903,234, 3,855,056, 3,841,943, 3,840,625, 3,790,435, 3,783,088, 3,551,194, 3,765,991, 3,758,661, 3,738,904, 3,551,538 and 2,971,858.
As an alternative to mechanically stretching the synthetic papers to produce microvoids, U.S. Pat. No. 3,840,625 discloses a method which relies upon elution and coagulation to deposit fine porous structures on the exterior surface to generate a paper-like film which can be used as a paper substitute. More particularly, U.S. Pat. No. 3,840,625 provides a process for producing a synthetic paper by first forming a film consisting essentially of an ethylene synthetic resin, a styrene resin and an inorganic filler, followed by dissolving the styrene resin with a first liquid and then precipitating the dissolved styrene resin with a second liquid to deposit the precipitated styrene resin onto the surface of the treated film. For other examples of synthetic papers which are formed by elution, see U.S. Pat. Nos. 3,855,056 and 3,551,538.
As another alternative method, U.S. Pat. No. 4,097,645 discloses a synthetic paper formed by coating a previously stretched olefin resin film containing a filler with an ethyleneimine adduct of a polyaminepolyamide for improving ink adhesion. For examples of other coated synthetic papers, see U.S. Pat. Nos. 4,705,719 and 4,340,639.
In yet another alternative method, U.S. Pat. No. 3,922,427 discloses a stretched synthetic paper-like film onto which an ethylenically unsaturated carboxylic acid, its anhydride, ester or amide has been graft copolymerized to produce a synthetic paper.
In still a further alternative method, U.S. Pat. No. 3,841,943 discloses a synthetic paper produced by laminating a paper-like film formed with a thermoplastic resin and an inorganic filler onto a previously stretched thermoplastic film, and then stretching the laminate.
In still a further alternative method, U.S. Pat. No. 3,553,302 discloses a synthetic paper produced by hydrating a sheet formed with a polyolefin resin containing gypsum, calcium sulfate hemihydrate and soluble anhydrite to impart printable properties to the film.
In addition to the above-described methods for producing synthetic papers, there are still other methods for obtaining synthetic papers. For example, a synthetic resin can be formed into staples or filaments which are interentangled and thus produced into a thin synthetic paper, as in the case of conventional pulp paper. Foamable styrene resins can be extruded into a thin sheet form to obtain a paper-like foam styrene resin film. Synthetic papers can also be obtained by flowing and spreading a solvent-dissolved synthetic resin onto a film to obtain a multi-cellular surface-roughened paper-like film. Still further, synthetic papers can be produced by coating the surface of a synthetic resin film with an inorganic or organic filler with the use of a binder or by embedding the same by means of heat and pressure. Alternatively, there are methods for obtaining synthetic papers by roughening the surface of synthetic resin films either by treating the surfaces with solvents or by treating them electrically, chemically and/or mechanically.
While paper-like thermoplastic synthetic resin films obtained by such known methods may possess certain favorable properties, such as water resistance, as compared with conventional paper made from pulp, their printability and paper-like properties are not only sometimes unsatisfactory, they may even change with the passage of time due to the complex manufacturing processes used to produce such paper-like films. Regardless, even if the paper-like films produced heretofore do not loose their favorable properties, they nevertheless are expensive to produce as a result of the complexities, additional components and additional processing steps associated with the manufacturing processes utilized hitherto to produce the synthetic paper substitutes.
Consequently, there is a demand in the industry for a synthetic paper substitute which has a wide range of utility and good paper-like characteristics including good receptivity and retentivity for writing materials, such as ink, pencil lead, paint and the like. Moreover, there is a demand in the industry for a synthetic paper substitute which can be manufactured simply and inexpensively.